In a wavelength division multiplexed (WDM) optical network, optical signals at a plurality of wavelengths are encoded with digital streams of information. These encoded optical signals, or “wavelength channels”, are combined and transmitted through a series of spans of optical fiber. At a receiver end, the wavelength channels are separated and detected by optical receivers.
In a reconfigurable WDM optical network, wavelength channels can be added or dropped at nodes of the network, using so-called reconfigurable optical add/drop multiplexors, or ROADMs. Nowadays, adding and dropping of wavelength channels can be done dynamically, in response to fluctuating data bandwidth requirements between various network nodes. From the network architecture standpoint, it is preferable that the ROADMs capability to dynamically add or drop wavelength channels be independent on wavelength channels presently used. This feature, called “colorless” add/drop capability of a ROADM, is highly desirable. It is further preferable that a ROADM can add and drop wavelength channels arriving from or going to a plurality of directions, without any limitations or contentions. This last feature of a ROADM is called “directionless”.
ROADMs allow flexible and wavelength-selective all-optical routing of wavelength channels at fiber optic network nodes. Colorless-directionless ROADMs further allow locally terminated channels to be tuned to different wavelengths and optically routed to a desired outbound direction in an automated fashion.
A conventional colorless-directionless ROADM 100 is illustrated in FIG. 1. The ROADM 100 includes three inbound wavelength-selective switches (WSS) 101, 102, and 103, for receiving inbound WDM optical signals 111 from West, North, and East directions, respectively; three outbound WSS 104, 105, and 106, for sending outbound WDM optical signals 112 to West, North, and East directions, respectively; and local add 113 and drop 114 switch sections for adding and dropping “local” wavelength channels, respectively. A WSS is an optical switch that can independently switch individual wavelength channels from any of its input port(s) to any of its output port(s).
Each of the add 113 and drop 114 switch sections include two pairs of WSS. Although one pair can be used, two pairs are preferable for redundancy purposes. In the add path, each pair includes a M×1 WSS 115 coupled to an 1×N WSS 116, for independent switching of any locally generated M wavelength channel to propagate in any of the West, North, and East outbound directions. In the drop path, each pair includes a N×1 WSS 117 coupled to an 1×M WSS 118, for independent switching of any wavelength channel arriving from the West, North, and East inbound directions to be detected by local optical receivers, not shown.
In operation, the M×1 WSS 115 select and combine wavelength channels to be added, and the 1×N WSS 116 route the selected wavelength channels to the desired outbound directions West, North, and East, via the corresponding WSS 104, 105, and 106. The ROADM 100 has a total of fourteen WSS, which results in a high device cost and a high optical losses. One could in principle replace the M×1 WSS 115 with optical combiners and 1×N WSS 116 with optical splitters, but that would increase optical losses even further, especially for high M, N port count, as well as potentially increase optical crosstalk.
Referring to FIG. 2 with further reference to FIG. 1, a prior-art ROADM 200 of FIG. 2 is similar to the prior-art ROADM 100 of FIG. 1. Inbound West (W) WSS 201, North (N) WSS 202, and East (E) WSS 203 correspond to the inbound WSS 101, 102, and 103 of the ROADM 100 of FIG. 1, respectively; and outbound West (W) WSS 204, North (N) WSS 205, and East (E) WSS 206 correspond to the outbound WSS 104, 105, and 106 of the ROADM 100 of FIG. 1, respectively. Optical splitters can be used in place of the inbound WSS 201 to 203, albeit at a cost of an increased insertion loss and/or crosstalk.
One difference of the ROADM 200 of FIG. 2 is that a pair of M×N WSS 215 and a pair of N×M WSS 216 are used in local add 213 and drop 214 switch sections, respectively. The M×N WSS 215 (FIG. 2) are used each in place of the M×1 WSS 115 coupled to the 1×N WSS 116 (FIG. 1); and the N×M WSS 216 (FIG. 2) are used each in place of the N×1 WSS 117 coupled to the 1×M WSS 118 (FIG. 1), for local add/drop colorless and directionless switching.
A drawback of the ROADM 200 of FIG. 2, limiting its practical use, is that presently available implementations for the M×N WSS 215 or N×M WSS 216 (the two have a same construction, because WSS are bidirectional devices) are quite complex and costly. Most suggested implementations for the M×N WSS 215 or N×M WSS 216 require multiple stages of switching and/or complex two-dimensional arrays of switching elements. By way of example, Colbourne in U.S. Pat. Nos. 8,045,854 and 8,300,995; Lalonde et al. in U.S. Pat. No. 7,106,966; Colbourne et al. in U.S. Pat. No. 8,233,794; Wisseman in U.S. Pat. No. 8,111,995; and Atlas et al. in US Patent Application Publication 2012/0027408, disclose such M×N wavelength-selective optical switches.